Investigation of the Influence of Correlations on Photovoltaic Energy Conversion in Hot Polaron Solar Cells

Abstract

In semiconductor single junction solar cells fundamental limitations are given by the two major intrinsic loss mechanisms: a) inability to absorb photons with energies below the band and b) thermalization of photon energies exceeding the band gap. The development of third-generation solar cells aims to produce highly efficient cells exceeding the SQ limit through new device and material design strategies such as hot carrier solar cells. The concept for hot carrier cells consists of two principal aspects: a) preventing hot charge carriers from thermalization to the band edge by slowing down or eliminating charge carrier cooling and b) fast extraction of hot carriers prior to thermalization process. Within this thesis, highly correlated perovskite manganite oxides are proposed as a model system for hot carrier solar cells, since it provides insights into new pathways and mechanisms for photovoltaic energy conversion beyond conventional semiconductor systems. $Pr_{1-x}Ca_{x}MnO_3$ (PCMO) x = 0.1 and a 2D layered $Pr_{0.5}Ca_{1.5}MnO_4$ (RP PCMO) are investigated as model systems. This enabled the establishment of a new type of phonon bottleneck principle for hot polaron manganite perovskites solar cells. The slowing down of carrier relaxation and the emergence of long-living states is given through strong coupling of electrons to cooperative lattice modes. In comparison to the 3D $Pr_{1-x}Ca_{x}MnO_3$ $x$ = 0.34 with a transition to the charge and orbital order phase at about 220 K, the epitaxial 2D RP PCMO thin films show a charge order transition temperature above room temperature. Moreover, the RP PCMO exhibits photovoltaic energy conversion above room temperature as well. Consequently, proof of principle for extending the hot polaron photovoltaics to a room temperature device has been acquired. The lightly doped 3D PCMO x = 0.1 exhibits a purely orbital order phase. Therefore, this material enables the extension of hot polaron photovoltaics to a different type of order. Additionally, according to the phase diagram by Jirak, the purely orbital order phase exists up to 800--1100 K; therefore, it offers a second model system for polaronic room temperature photovoltaics. However, the onset of photovoltaic energy conversion is experimentally observed only well below room temperature. A readjustment of the phase diagram for low doping is proposed and the concept of spontaneous orbital order with long-living states below room temperature and an induced orbital order of Jahn--Teller distortions at higher temperatures is discussed. This additional phase transition and the change of the phase diagram is supported by several anomalies in various physical properties, e.g., transport, magnetic, optic, ultra-fast transient excitation probe studies; change in lattice constant; and finite temperature simulations based on tight binding. The mechanism for photovoltaic energy conversion in hot polaron systems differs in nature from conventional semiconductors.Therefore, the dependence of characteristic parameters, such as the open circuit voltage and the short circuit current density on illumination conditions, including photon energy and power density, are analysed for the orbital order system. The interplay and influences of correlation phenomena such as phase transition orbital order phase and kinetic contributions are discussed.